The SConscript in src/core/sal/gem5 is now generated via CMake
(SConscript.in). No more hardcoded relative paths -> Fail* build
folder can now be anywhere. Experiment and Plugin libraries are now
set automagically (using ${EXPERIMENTS_ACTIVATED} /
${PLUGINS_ACTIVATED})
Generated SConscript now resides in binary dir.
Change-Id: I1bf2e17c83c95ffdcf6801c02481064fcb63bfb0